1) Field of the Invention
The invention relates to modified polvinyl acetals, a process for their preparation and their use.
2) Background Art
The preparation of polyvinyl acetals from the corresponding polyvinyl alcohols by polymer-analogous reaction with the corresponding aldehydes has been known since 1924, and many aldehydes have been used for preparing the corresponding polyvinyl acetals since then. Polyvinyl acetals are prepared in a three-stage process (polyvinyl acetate .fwdarw. polyvinyl alcohol .fwdarw. polyvinyl acetal), resulting in products which contain not only vinyl acetal groups but also vinyl alcohol and vinyl acetate units. In the following, modified polyvinyl acetals are those which contain further units in addition to the three units mentioned above. In particular, polyvinyl formal, polyvinyl acetal and polvinyl butyral have achieved commercial importance.
The largest application area for polyvinyl acetals is the production of safety glass for automobile construction and in architecture, where plasticized polyvinyl butyral films are used as intermediate layers in panes of glass. Modified polyvinyl butyrals have also been proposed for this application, for example the polyvinyl butyrals containing sulfonate groups described in EP-A-368832.
Polvinyl butyrals modified with amino groups are known from EP-A 461399. These are used as precipitants.
A further application area for polyvinyl butyrals is their use in anticorrosion coatings. For this purpose, EP-A 505939 (U.S. Pat. No. 5,236,999) proposes the use of modified polyvinyl butyrals which are crosslinkable due to the acetal functions derived from aldehydes containing keto groups.
Owing to, inter alia, their good pigment binding power, polyvinyl butyrals are also used as binders in surface coatings and specifically in printing inks. This application requires that the organic solutions of the polyvinyl butyrals should have a solution viscosity which is as low as possible in order to be able to produce inks having a high solids content at as high a binder content as possible.
In the prior art, the solution viscosity of polyvinyl butyral is regulated via the molecular weight by using polyvinyl acetates having a low molecular weight or degree of polymerization as starting materials for preparing the polyvinyl butyral. The use of low molecular weight polvinyl butyrals as carrier material in pigment preparations for printing inks is described, for example, in DE-B 2643076 (U.S. Pat. No. 4,130,539). A disadvantage here is that considerable problems arise in the preparation of low molecular weight polyvinyl butyrals by acetal formation from low molecular weight polvinyl alcohols, since solutions of low molecular weight, fully saponified polyvinyl alcohols tend, inter alia, to gel and therefore have to be handled at temperatures of at least 50.degree. C.